Fillers are typically added to paper during the paper making process to improve the optical properties of the paper, such as brightness. In acid papermaking, which is often used for making paper from a groundwood pulp, the pH of the paper stock is generally substantially less than 7, and is normally in the range of from about 4 to about 6. In comparison, in neutral papermaking, the pH is normally maintained in the range of from about 6.8 to about 7.5. Therefore, in acid papermaking and under certain conditions in neutral papermaking, a filler material that is stable and resistant to attack by acid under weakly acidic to acidic conditions is required.
Titanium dioxide and calcined clay have often been used as fillers in neutral and acidic paper. However, these materials, and titanium dioxide in particular, are expensive, thus adding to the cost of the paper.
Both ground natural and precipitated calcium carbonate are used as a filler material for alkaline paper, which is typically produced from a paper stock having a pH of about 8. The resulting paper has enhanced optical properties without the cost of more expensive fillers. However, calcium carbonate normally decomposes in acidic aqueous media, and, thus, generally cannot be used as a filler in acid paper unless the calcium carbonate filler is acid-stabilized, that is, made resistant to decomposition at a pH of less than 7. Because alkaline conditions and alkaline fillers typically have a negative impact on the properties of groundwood paper, a calcium carbonate filler can only be used with a groundwood paper pulp if the calcium carbonate filler is acid-stabilized for use under acidic conditions.
Calcium carbonate filler materials, both precipitated and ground natural, have been treated or modified in the prior art for various reasons. In the case of precipitated calcium carbonate, the treatment or modification has been performed both during and after the precipitation process. Treatments include coating calcium carbonate particles with a non-reactive material, such as a polysaccharide, fatty acid, or gum material, and treating calcium carbonate particles or a slurry of calcium carbonate with various weak acids and bases, salts, and chelating agents.
In particular, U.S. Pat. No. 4,240,870 discloses the addition of at least one phosphoric acid, such as orthophosphoric acid, metaphosphoric acid, tripolyphosphoric acid, pyrophosphoric acid, tetrapolyphosphoric acid, and hexapolyphosphoric acid, or the sodium, potassium, or zinc salt of at least one phosphoric acid to an aqueous calcium hydroxide suspension as the first step of a multi-step process for the production of precipitated calcium carbonate. Similarly, U.S. Pat. No. 4,244,933 discloses the addition of one of the phosphoric acids or salts disclosed in U.S. Pat. No. 4,240,870 as an ingredient in either the first or second step of a multi-step process for the production of precipitated calcium carbonate.
U.S. Pat. No. 4,018,877 discloses the addition of a complex-forming agent, such as a polyphosphate, e.g., sodium hexametaphosphate, during or after the end of the first carbonation step in a multi-step process for the production of precipitated calcium carbonate. Preferably, the agent is added after the bulk of the calcium carbonate precipitation is complete. Alternatively, the complex forming agent is added during a subsequent ageing or carbonation step.
U.S. Pat. No. 4,157,379 disclose the addition of a soluble metal salt, such as an alkali metal phosphate, after the primary carbonation of an aqueous suspension of calcium hydroxide.
Laid-open Japanese Patent Application No. 090821/60 discloses a process for the preparation of calcium carbonate in which a condensed phosphoric acid or salt thereof is added to the viscous gelatinous emulsion formed from the carbonation of an aqueous dispersion of calcium hydroxide. The condensed phosphoric acid may be hexametaphosphoric acid, pyrophosphoric acid, tripolyphosphoric acid, polyphosphoric acid or ultraphosphoric acid. Laid-open Japanese Patent Application No. 090822/60 discloses the same process with the addition of a magnesium-containing compound in the dispersion of calcium carbonate.
However, all of the references cited above disclose processes that require the addition of a phosphoric acid or phosphate salt during a process for the production of precipitated calcium carbonate.
U.S. Pat. No. 4,793,985 discloses the addition of a dispersing agent, such as a water soluble salt of polyphosphoric acid or a phosphate, particularly, sodium hexametaphosphate, to a slurry of ground calcium carbonate to improve the distribution of solids within the liquid, as part of a process for producing an ultrafine calcium carbonate.
U.S. Pat. No. 4,610,801 discloses the use of polyphosphoric or polyphosphates in a slurry of mineral particles, such as calcium carbonate, in waste treatment.
U.S. Pat. No. 4,219,590 discloses the treatment of calcium carbonate, having an average particle diameter of no more than 20 .mu.m, with an acid gas that is capable of reaction with calcium carbonate.
Japanese Patent No. 030812/82 discloses improving the surface of calcium carbonate particles by adding an aqueous solution of a condensed phosphate, such as a metaphosphate or pyrophosphate to an aqueous calcium carbonate suspension. As a result, the calcium carbonate particles have a resistance to acid, and the pH of the particles is reduced by 0.1 to 5.0.
U.S. Pat. Nos. 5,043,017 and 5,156,719 disclose an acid-stabilized calcium carbonate for use as a filler in neutral to weakly acidic paper. These patents state that the acid-stabilized calcium carbonate is able to resist degradation under mildly acidic conditions due to a buffering action between an absorbed or reacted calcium-chelating agent or conjugate base on the surface of the calcium carbonate and a weak acid in solution in the calcium carbonate slurry.
U.S. Pat. No. 5,505,819 discloses a process for producing neutral paper from wood-containing pulp and a natural calcium carbonate filler. The pH of the stock is maintained in the range of 6.7 to 7.3 by the addition of a pH reducing and buffering agent, preferably phosphoric acid.
U.S. Pat. No. 5,531,821 discloses an acid resistant calcium carbonate filler for use in making neutral to weakly acidic paper, comprising a mixture of calcium carbonate and about 0.5 to 10 percent, based on the dry weight of calcium carbonate, of a cationic salt and an anionic salt.
There still remains a need for improved acid-stabilized calcium carbonate materials and methods for producing such materials for use in acid papermaking. The present invention provides one solution to satisfy this need.